Apparatus for producing and purifying plasma



APPARATUS FOR PRODUCING AND PURIFYING PLASMA Filed April 14, 1964 Aug.9, 1966 W- R BAKER ET AL 2 Sheets-Sheet 1 :Prism ZmaO ATTORNEY Aug. 9,1966 W. R BAKER ET AL 3,265,583

APPARATUS FOR PRODUCING AND PURIFYING PLASMA 2 Sheets-Sheet.

Filed April 14. 1964 INVENTORS WILL/AM R. BAKER BY KLAUS HALBACHATTORNEY mission Filed Apr. 14, 1964, Ser. No. 359,803 12 Claims. (Cl.176-7) The present invention relates generally to magnetohydrodynamicplasma devices and more particularly to a device for generating,containing land heating an ionelec tron plasma of high purity.

The maximum degree of heating, and consequently the number of nuclearreactions, that may be obtained in a plasma heating device is severelylimited if even small quantities of impurities are present. Thus,various devices have been developed in which the generation andmaintenance of more pure plasma has been a major objective, theapparatus described in U.S. Patent No. 3,104,- 345 and in copending U.S.Application Serial No. 177,- 140 filed March 2, 1962 by William R.Baker, now Patent 3,156,623, dated November 10, 1964, being examples.

Most impurities enter a plasma during the formation of the plasma andimmediately after the plasma is produced as at such times the plasma isproximal to various electrodes associated with producing the plasma. Theenergetic plasma particles bombard the electrodes and release impuritiestherefrom.

The prior devices intended to produce pure plasma control only theinitial conditions under which the plasma is produced. The presentinvention also provides control over the initial conditions, but inaddition provides a means for removing impurities which enter a plasmasoon after it is produced but before it has left the vicinity of theelectrodes so that higher degree of nal plasma purity is obtainable.

In the present invention, the plasma is produced between a conductivecylinder and a short axial electrode disposed at one end thereof. Anaxially directed magnetic field and a radially directed electric eld areprovided between the electrode and the cylinder and the combined fieldscause charged particles to rotate around the center electrode incycloidal orbits. Considering now the means by which initial purity ismaximized, it will -be observed that for each particular ratio ofmagnetic to electric ield, the size of the orbit of every chargedparticle of a given m-ass will be equal. Heavier particles have orbitswith a larger radius than the orbits of lighter particles. Plasmas aregenerally comprised of very light elements such as isotopes of hydrogen,for instance. Therefore, if the spacing between the electrode and theencircling cylinder is slightly larger than the orbital radius of theheaviest desired particle in the plasma, all heavier particles, whichare considered to be impurities, will strike the encircling cylinder andthus be removed from the plasma.

The purity of the plasma is further maintained by control of stillanother condition. The electrode projects a short distance into one endof the tube. The portions of the electric lield lines near the tip ofthe axial electrode are parallel with the magnetic iield lines, thuspermitting plasma particles to accelerate along the electric ield lineswithout restraint by the magnetic lield. In previous apparatus suchparticles `strike the electrode and release contaminants into theplasma. In the present invention, the above process is minimized byeliminating the electric iield when the plasma is adjacent the tip ofthe axial electrode, this being effected by use of a novel axialelectrode conguration and by control of the electric ield power supply.

Patented August 9, 1966 The initial purity of the plasma is relativelyhigh owing to the means described above. However, in spite of suchmeans, some impurities subsequently enter the plasma as a result ofelectrode bombardment. Such impurities are removed from the plasma inthe present invention by producing the plasma at one end of the longconductive cylinder and then drifting the plasma in a cluster or cloudtoward the final containment and heating region at the opposite end ofthe tube. The energy imparted to the plasma particles during plasmacreation is generally much higher than that of the subsequentcontamination particles. Therefore, as the plasma progresses along thetube, the contaminating particles tend to lag behind the plasma cloudand are separated therefrom. After separation, a magnetic barrier iieldis rapidly produced between the plasma cloud and the impurities toprevent the impurities from following the plasma into the plasmacontainment magnetic eld.

It is an object of the present invention to provide a mre eiicient meansfor producing and maintaining a highly pure plasma.

It is an object of the invention to provide a magnetohydrodynamic devicecharacterized by intense plasma heating whereby .a greater rate ofnuclear interactions may be obtained.

It is an object of the invention to provide means for avoidingcontamination during the generation of a plasma.

It is another object of the invention to provide a means for separatingout plasma contaminants prior to injection of the plasma into a trappingregion.

It is another object of the present invention to provide a means forcontrolling the electric ield conguration adjacent the electrodes of aplasma generator so that the production of contaminants is minimized.

It is another object of the present invention to produce a puriliedplasma by providing a suitable combination of electric and magneticfields *between a spaced pair of electrodes so that orbiting plasmaparticles may pass therebetween while heavy impurity particles whichhave a larger orbital radius are removed from the plasma.

It is another object of the present invention to provide -a means forremoving impurities from a plasma by utilizing differences in the drifttimes between two distant points for the plasma and the impurities.

The invention will be better understood by reference to the followingspecilication Itogether with the accompany ing drawings of which:

FIGURE 1 is a broken-out view taken at right angles to the axis of amagnetohydrodynamic device embodying the present invention,

FIGURES 2 to 5 are simplified views of the invention showing theposition of lthe plasma during progressive stages of the operation, and

FIGURE 6 is a simplified View of the plasma generating electrodes in theinvention with an indication of the electric field coniigurationtherebetween.

Referring now to FIGURE l, there is shown a plasma device 11 which isformed in part by a long, electrically conductive cylindrical tube 12with a length at least ten times greater than the diameter. The ends ofthe tube 12 are closed by insulative discs 13 4and 14, which are boltedand sealed, for example, by O-rings 16 so that all gasses can beevacuated from the tube 12. Gasses are removed by a vacuum pump 20communicated to the interior of the tube 12 through the disc 14. Theopposite disc 13 is provided with a central aperture 17 through which anaxial electrode 18 extends for a short distance into the tube 12 alongthe axis of the tube. A plurality of openings 15 are distributed aroundthe circumference of the axal electrode 18 near the inner end thereofand a rapidly opening gas valve 19 is disposed within the elecltrode 18to control the release of gas from the openings. Thus gas released bythe valve 19 may readily diffuse into the space within the tube 12through openings 15. The rapidly opening valve 19 releases a measuredquantity of gas in a sudden burst or puff in response to an electricalcontrol signal, the valve being of the quick opening type described inU.S. Patents, 3,021,272 or 3,096,269. In the present invention, eachvalve control signal is provided by a central valve and switch control21. A gas such as deuterium or tritium is provided from a gas supply 22through a gas line 23 to the valve 19. A conventional shutoff gas valve24 is also generally included in the gas line 23.

A conductive rod-shaped electric fieid shaping element 26 is afiixed tothe inner end of the axial electrode 18 and projects therefrom along theaxis of the tubem12 toward the insulative disc 14. The end of theelement 26 is rounded to avoid field emission and the diameter of theelement is approximately ten percent of the electrode 18 diameter. Thelength of :the field shaping element 26 and the axial electrode 18combined is generally less than ten percent of the total length Iof thetube 12. Because 'of the small diameter `of the element 26, many chargedparticles which ,are accelerated toward the ele- Ament by the electricalfield will miss it, thus avoiding release lof impurities therefrom. Thefunction of the element 26 in controlling the electric field is furtherdiscussed hereinafter.

To form a plasma, an axially directed magnetic field and a radialelectric field are established between the electrode 18 and the tube 12to effect the ionization of thle gas released by valve 19. Power forproducing the electri-c fie-ld is Iprovided by a conventional highvoltage and current power supply 27, shown in block form. Since powersupplies generally have a high output impedance, a sudden large surge ofcurrent cannot be produced as is required for the present invention.Thererfore, the high voltage supply 27 is used to charge a capacitorIbank 28 over a relatively 'long time period. After being charged, thecapacitor bank 28 is suddenly discharged by closing a normally openswitch 29, usually an ignitron twhich, when closed, connects one side ofthe capacitor bank 28 to the axial electrode 18 through a low ohmagedamping resistor 31, the purpose of which will be discussed later indetail. The tube 12 -is grounded as is the opposite side of capacitorbank 28.

After burn out, defined as complete ionization of the gas in the tube12, the electric field is removed as rapidly as possible by closing anormally ropen crowbar switch 32 connected 4from ground to the junctureof the damping resistor 31 and the normally open switch 29. The timing'of the closing of crowbar switch 32 provides for complete removal ofthe electric field before the plasma passes the end of the field shapingelement 26. The crowbar switch 32, also generally an ignitron, thusprovides a short circuit across the capacitor bank 28, but oscillatorycurrent Vreversal in the power circuitry is prevented by the criticaldamping resistor 31. Such resistor 31 generally will have a resistanceof less than one ohm, the value being found lby taking twice the squareroot of `the net circuit inductance divided by the total capacitance ofcapacitor 28.

(Resistance yof resistor 31 =2\/net circuit inductance/ capacitance ofbank 28) netic mirror field is created Within the tube 12 shaped asindicated by dashed lines 37.

A second magnetic field 38 is provided by a plurality of drift fieldcoils 39 disposed coaxially along nearly the entire length of the tube12 except for the end portion where the magnetic mirror coils 33 and 34are disposed. A drift field power supply 41 provides excitation for thecoils 39 to form an axially directed magnetic field 40 within thecontrol region of tube 12 essentially las indicated by dashed lines. Aspart of the drift field 40, a low ratio magnetic mirror 42 from a lowratio mirror coil 45 is provided at the region of the gas openings 15near the end of axial electrode 18. When energized from a power supply43 the resultant mirror field 42 is five to ten percent more intensethan the remainder of the drift field 49. The purpose of such low ratiomirror 42 is to impel the plasma toward the magnetic mirror containmentfield 37 The drift field 40 lguides the plasma from the region of theaxial electrode 18 toward 'the containment field 37. While the low ratiomirror supply 43 is indicated as having a steady state 'output current,it is equally feasible as a variation to operate supply 43 in a pulsemode' controlled by a signal from control 21.

To aid in maintaining plasma purity, a spacer ring 44 is disposedcoaxially against the inside walil of the tube 12 around the gas opening15 :so that such inside wall, with the spacer ring, conformsapproximately to the shape of the low ratio mirror 42. The ring 44prevents plasma from reaching the outermost magnetic fiux lof the lowratio mirror 42 which tiux intersects the wall of the tube 12 at thestart of the drift field 38 region. Thus lsuch plasma cannotsubsequently release impurities by striking the tube 12 wall. Suchspacer ring 44 also determines the size of the gap within which theplasma is produced. As previously discussed, impurities are heavier thanthe plasma particles, and have a larger :orbit around the centerelectrode 18 than the plasma particles. Owing to the size of the gap,4only the lighter plasma particles can 'orbit without lstriking thespacer ring 44. Thus the heavy impurities are removed.

A third magnetic field is provided in a pulsed manner to aid inseparating the purified plasma from impurities. An annular gate coil 46,disposed around the tube 12 near the plasma entrance end of the magneticcontainment field 37, is connected to the gate power supply 47 whichprovides a pulse of current to the coil. The gate coil 46 has relativelylow inductance compared to the mirror field coil 33 so that a magneticfield is very rapidly created in the gate coil 46 while the magneticfield of coil 33 builds up much more slowly. The time at which the gatepower supply 47 is actuated is related to the opening of the fast gasvalve l19 by an appropriately delayed signal from the valve and switchcontrol 21 such that high purity plasma is trapped in the mirrorcontainment field region 37 and later arriving impurities are rejectedby buildup of the intervening pulsed field of coil 46. Since, in movingalong tube 12, the impurity particles lag behind the plasma, gate supply47 is pulsed onafter the plasma passes the region encircled by the gatecoil 46, but before the impurities have arrived at such region. The gatemagnetic field of coil 46 reflects the late arriving impurities backtoward the region of the axial electrode 13 While the purified plasmapasses on into the containment mirror field 37. Thus the ygate coil 46provides a pulsed magnetic field which rapidly rises and decays.However, during the gate pulse the mirror field of the coil 33 has timeto build up and take over the function of containing the plasma.

It is possible to combine the functions of the gate coil 46 and themirror coil 33 into that of a single coil if such single coil tand theassociated power supply are adapted to provide a magnetic field which isvery rapidly pulsed on and remains on. However the use of both the gatecoil 46 and the field coil 33 significantly lowers the cost ofconstruction.

The presence of a rapidly pulsed gate field from coil 46 requires thatthe adjacent containment field coil 33 and nearby drift field coils 39each be shielded by A.an vouter conductive cover 4S formed of a materialsuch as copper sheet. Such covers 4S prevent a rapidly expanding pulsedmagnetic field from the gate coil 46 from inducing unduly high andpossibly damaging currents in the adjace-nt coils 33 and 39. A rapidlychanging magnetic field cannot penetrate through the copper shields 48,but the more slowly changing magnetic fields of the field coil 33 andthe steady state fields from the drift coils 39 are not affectedappreciably by such shields. The shields 48 must each be discontinuousso as not to constitute a shorted turn.

Considering now the operation of the invention, assume first that thecontainment magnetic mirror coil 34, the drift magnetic coils 39, andthe low ratio magnetic mirror coil 45 are all actuated. The gate powersupply 47 is not initially energized. The switch control 21 provides ana output signal to open the valve 19 and release gas through theapertures of axial electrode 1S. It is necessary that the gas not beallowed to diffuse along any substantial portion of the tube 12, butrather that the gas be ionized immediately so that a well defined bodyof plasma is created. Thus the switch 29 is closed by a b signal fromthe switch control 21 nearly simultaneously with the release of gas fromthe valve 19 so that an electric field is already established betweenthe axial electrode and the tube 12 by the time the gas has diffusedsufficiently that a discharge can form. The gas, therefore, can functionas its own switch by diffusing out to the wall of tube 12 and providinga current path between the tube 12 and electrode 1S. Utilizing sucheffect, switch 29 may be eliminated in some instances.

Referring now to FIGURE 2 in conjunction with FIG- URE 1, there is showna simplified diagrammatic view of salient elements of the invention inwhich the initial configuration of the plasma cloud is shown. The plasma51 in FIGURE 2 and the subsequent FIGURES 3 to 5 is indicated by shortcurved lines while impurities 52 are indicated by dots. The plasma 51 iscreated between the axial electrode 1S and the tube 12 and, as describedpreviously, impurities are filtered out by the orbital spacing betweenthe ring 44 and axial electrode 18. The low ratio magnetic mirror 42causes plasma 51 and the impurities 52 to drift toward the opposite endof the tube 12 owing to the well known repulsive :force exerted by amagnetic mirror. As soon as burn-out (complete ionization) occurs andbefore the plasma 51 has reached the end of the field shaping element26, the crowbar switch 32 (FIGURE 1) is closed to remove the electricfield as previously described. Such prompt `removal of the electricfield prevents, -to some extent, chargedparticles from being acceleratedat the field element 26 and releasing impurities therefrom.

Referring now to FIGURE 3 in conjunction with FIG- URE 1, the plasmacloud 51 then progresses along the tube 22 beyond the axial electrode18, the plasma being guided by the magnetic drift field 3S previouslydescribed. During this movement along tube 12 the plasma impurities S2tend to lag behind the plasma cloud 51.

Referring now to FIGURE 4 in conjunction with FIG- URE 1, the plasmacloud 51 subsequently passes through the gate coil 46, which is thenenergized as controlled by a signal d from valve and switch Control 21so that the impurities 52 are prevented from following the plasma cloud51 and are accelerated back toward the axial electrode 18. Thecontainment function is subsequently assumed by the mirror field fromthe field coil 33, as shown in FIGURE 5. After the plasma 51 is in thecontainment field 37, various further heating means, well known in theart, may be employed to cause nuclear interactions such as fusion of theplasma particles for example.

Referring now to FIGURE 6, the configuration of electric field 61between the field shaking element 26 of electrode 18 and the tube 12 isshown to clarify the action of element 26 with respect to preventingimpingement of plasma particles on electrode 18. The electric field 61extends radially between the tube 12 and the element 26, therefore theelectric field is transverse to the axial magnetic field in tube 12.Without the field element 26, the electric field in the region near theend of the axial element 18 would tend to be parallel to the magneticfield lines, since field lines emerge from a surface at a right angle.The disadvantage of having the electric field directed parallel to themagnetic field is that charged particles can accelerate along theelectric field lines without restraint by the magnetic field.Accelerated particles would thus strike the end of the electrode 18 andimpurities would be released into the plasma. However, with the fieldelement 26, the axial component in the electric field is largelyavoided. Before the plasma body reaches the free end of the element Z6,the electric field has been removed by the closing of the crowbar switch32, thus the electric field at the end of the element 26 does not existwhen the plasma body is adjacent thereto. If the element 26 were notprovided, the plasma would be within a zone having parallel electric andmagnetic field lines before the crowbar switch 32 could function. Thesmall diameter of the element 26 causes the particles which areaccelerated toward the element to miss it, thus avoiding release ofimpurities.

Accordingly, means are employed in the present invention to avoidcontamination of the plasma at the time it is formed with further meansbeing provided to remove impurities which unavoidably get into theplasma after it is formed and before it can be removed from the vicinityof the electrodes. The resultant plasma is thus relatively quite free ofimpurities and therefore may be heated to higher temperature.

While the invention has been disclosed with respect to a p-articularembodiment, it will be apparent to those skilled in the art thatnumerous variations and modifications may be made within the lspirit andscope of the invention and it is not intended to limit the inventionexcept as defined in the following claims.

What is claimed is:

1. In apparatus for producing and containing a very puremagnetohydrodynamic plasma, the combination comprising a pair of spacedapart coaxial field coils producing a first magnetic field having aplasma containment configuration, said first field being symmetricalabout a linear axis, a pair of spaced apart electrodes disposed inproximity to said axis at a position remote from said first magneticfield whereby a plasma drift space is provided therebetween, means formaintaining a vacuum in the region of said first field and saidelectrodes and the drift space therebetween, means providing a secondmagnetic field in the region of said electrodes and said drift space,

said second field being directed substantially parallel to said axis andbeing of greatest intensity in the region of said electrodes, means foradmitting gas to the region between said electrodes, an electrical powersupply connected across said electrodes, a third magnetic fieldgenerating means situated between said first field and said drift space,and a pulsed power supply coupled to said third field means to energizesaid third field means after said plasma has passed therethrough andprior to the arrival of trailing im-purities thereat.

2. Apparatus for producing and containing a very puremagnetohydrodynamic plasma as described in claim 1 wherein said thirdfield generating means comprises an annular coil disposed coaxially withrespect to said first field coils and spaced apart therefrom a distancewhich is small relative to the distance of said electrodes therefrom.

3. A magnetohydrodynamic device for creating a purified plasma,comprising, in combination, a long hermetically sealed tube having alength substantially greater than the diameter thereof and having afirst end and a second end, means for evacuating said tube, a hollowelectrode extending a small distance along the axis of said tube at saidfirst end thereof, said electrode having at least one gas aperture atthe inner end thereof, a rapidly opening gas valve disposed within saidelectrode for release of gas through said aperture, a gas sourceconnected to said valve, a first high current power supply connectedbetween said tube and said electrode, a pair of annular magnetic mirrorcoils disposed coaxially on said tube at said second end thereof, asecond power supply connected to said mirror coils, a drift eld coildisposed coaxially on said tube to provide an axially aligned magneticheld in the region thereof between said electrode and said pair ofmagnetic mirror coils, said drift field having a highest intensity inthe region between said first end of said tube and said apertures insaid electrode, a gate coil dispose: coaxially on said tube between saidpair of magnetic mirror coils and said electrode, and a pulse powersupply connected to said gate coil.

4. A magnetohydrodynamic device as described in claim 3, comprising thefurther combination of a valve and switch control providing forsequential operation of said valve and said pulse power supply wherebysaid gate coil is energized immediately after the passage of said plasmatherethrough and prior to passage of plasma irnpurities therethrough.

5. A magnetohydrodynamic apparatus for creating and purifying a plasma,comprising, in combination, a long gas tight tube having a first end anda second end, a vacuum pump communicated with said tube, an electrodedisposed at said first end of said tube and extending for a shortdistance along the axis thereof with a first end facing said second endof said tube, a rapidly opening gas valve disposed at the first end ofsaid electrode for releasing a burst of gas to produce said plasma, athin conductive rod afiixed at one end to said first end of saidelectrode and aligned along the axis of said tube, means impelling saidplasma toward said second end of said tube, a pair of magnetic mirrorfield coils disposed coaxially with respect to said tube at the secondend thereof, at least one drift field coil disposed coaxially withrespect to said tube and providing an axially aligned magnetic fieldtherein, a coil disposed coaxially with respect to said tube betweensaid electrode and said pair of mirror field coils, and a pulse powersupply connected to said gate coil whereby impurities which follow saidplasma along said tube may be stopped from entering said mirror field.

6. A magnetohydrodynamic apparatus as described in claim 5, wherein theinside wall of said tube is constricted in the region of the first endof said electrode and has a configuration substantially following thecurvature of said drift field in the region adjacent said electrode.

7. `In a magnetohydrodynamic device for creating substantially pureplasma, the combination comprising a long hermetically sealed tubehaving a first end and a second end, a vacuum pump coupled to said tube,an electrode disposed at said first end of said tube along the axisthereof and having an inner end projecting toward said second end ofsaid tube, a relatively thin conductive rod secured to the inner end ofsaid electrode and projecting therefrom toward the second end of saidtube along the axis thereof, means producing an axially directedmagnetic field within said tube, a power supply connected between saidtube and said center electrode to produce an electric field from saidrod and said electrode to said tube, means for impelling said plasmafrom said first end of said tube toward said second end of said tube,means releasing a burst of gas in said first end of said tube to form aplasma thereat, a control circuit for disconnecting said power supply toeliminate said electric field a timed interval after operation of saidgas release means whereby said electric field is absent when said plasmapasses the free end of said rod, means producing a magnetic plasmacontainment field at the second end of said tube, and a rapidly closinggate means of a type impervious to the passage of charged particles,said gate means being disposed along said tube between said electrodeand said plasma containment field and being operative an interval afteroperation of said gas release means whereby impurities which follow saidplasma along said tube are prevented from entering said containmentfield.

8. In a magnetohydrodynamic device for creating a pure charged particleplasma, the combination comprising a 4long conductive tube having firstand second ends, an insulative closure `at said first end of said tube,an electrode having one end afiixed to the center of said closure andhaving an apertured inner end which projects along the axis of said tubetoward said second end thereof, said electrode being short relative tosaid tube, a plasma containment magnetic eld producing means disposed atsaid second end of said tube, means emitting a burst of gas through saidapertured end of said electrode, a plasma drift field producing meansdisposed between said first end of said tube and said containment fieldmeans, means providing a low intensity magnetic mirror in said firstfield in the region between said inner end of said electrode and saidclosure, a high voltage power supply connected between said tube andsaid electrode, the voltage output of said power supply having a valuewhereby the heaviest of said charged plasma particles are guided inorbits having a maximum. radius slightly less than the spacing betweensaid electrode and said tube, a rod secured to said apertured end ofsaid electrode and projected axially therefrom ltoward said second endof said tube, means de-activating said power supply when said plasmaparticles approach the end of said rod, and a pulsed gate magnetic fieldgenerating coil disposed coaxially with respect to said tube betweensaid containment magnetic field and said electrode.

9. A magnetohydrodynamic device as described in claim 8, furthercharacterized in that said means de-activating said power supplycomprises a valve and power supply timing circuit, said timing circuitbeing of the type producing -time related output signals, a first ofsaid output signals causing said gas emitting means to be activated,followed in time by a second of said signals initiating operation ofsaid power supply deactivating means.

l0. A magnetohydrodynamic device as described in claim 8, furthercharacterized in that said means deactivating said power supply is anormally open switch connected across the output of said power supply,and wherein a low ohmage damping resistor is connected between saidpower supply and said electrode.

11. In a magnetohydrodynamic device for producing a purified plasma ofenergetic particles, the combination comprising a long conductivecylinder, an axial electrode disposed within one end of said cylinderand having a first end facing `the opposite end of said cylinder, theradial distance between said electrode and said cylinder slightlyexceeding the orbital radius of said particles therebetween, means forreleasing gas from said electrode to produce said plasma, meansproviding `a magnetic field axially through said cylinder, meansimpelling said plasma toward said opposite end of said cylinder alongsaid magnetic field, Ia plasma containment Vmeans at said opposite endof said cylinder, a conductive axially disposed rod `afiixed at a firstend to said first end of said electrode and having a second end directedtoward said opposite end of said cylinder, a high voltage power supplyconnected from said electrode to said cylinder, and means de-energizingsaid power supply when said plasma is adjacent said second end of saidrod.

12. In a magnetohydrodynamic device for producing a purified plasma ofcharged particles the combination comprising a long evacuated conductivecylinder, an axial electrode disposed within said cylinder at one endthereof and radially separated therefrom by a gap of predeterminedradial depth, means producing a radial electric field between saidcylinder and said electrode, means producing an axially directedmagnetic field through said tube,

References Cited by the Examiner UNlTED STATES PATENTS 3,021,272 2/1962Baker 176-6 3,()96,269 7/1963 Halbach et al 176-6 3,104,345 9/1963Wilcox et al 176-7 3,156,623 11/1964 Baker et al 176-8 REUBEN EPSTEN,Primary Examiner'.

1. IN APPARATUS FOR PRODUCING AND CONTAINING A VERY PUREMAGNETOHYDRODYNAMIC PLASMA, THE COMBINATION COMPRISING A PAIR OF SPACEDAPART COAXIAL FIELD COILS PRODUCING A FIRST MAGNETIC FIELD HAVING APLASMA CONTAINMENT CONFIGURATION, SAID FIRST FIELD BEING SYMMETRICALAOUT A LINEAR AXIS, A PAIR OF SPACED APART ELECTRODES DISPOSED INPROXIMITY TO SAID AXID AT A POSITION REMOTE FROM SAID FIRST MAGNETICFIELD WHEREBY A PLASMA DRIFT SPACE IS PROVIDED THEREBETWEEN, MEANS FORMAINTAINING A VACUUM IN THE REGION OF SAID FIRST FIELD AND SAIDELECTRODE AND THE DRIFT SPACE THEREBETWEEN, MEANS PROVIDING A SECONDMAGNETIC FILED IN THE REGION OF SAID ELECTRODES AND SAID DRIFT SPACE,SAID SECOND FIELD BEING DIRECTED SUBSTANTIALLY PARALLEL TO SAID AXIS ANDBEING OF GREATEST INTENSITY IN THE REGION OF SAID ELECTRODES, MEANS FORADMITTING GAS TO THE REGION BETWEEN SAID ELECTRODES, AN ELECTRICAL POWERSUPPLY CONNECTED ACROSS SAID ELECTRODES, A THIRD MAGNETIC FIELDGENERATING MEANS SITUATED BETWEEN SAID FIRST FIELD AND SAID DRIFT SPACE,AND A PULSED POWER SUPPLY COUPLED TO SAID THIRD FIELD MEANS TO ENERGIZESAID THIRD FIELD MEANS AFTER SAID PLASMA HAS PASSED THERETHROUGH ANDPRIOR TO THE ARRIVAL OF TRAILING IMPURITIES THEREAT.